This invention relates to an ion-exchange material. More specifically this invention relates to an ion-exchange material which is selective for cesium and other ions and which is compatible with concrete for the long term storage of radioactive nuclear wastes. This invention also relates to a method of fixing cesium and other ions for long-term storage.
Cooling water flowing through nuclear power reactors becomes radioactive over a period of time due to the presence of radioactive cesium, strontium, cobalt and other ions. Situations also arise in nuclear power plants which result in the presence of large quantities of radioactive cesium in the cooling water. At the present time, these radioactive ions are generally removed by passing the cooling water through an ion-exchanger which removes the radioactive ions from the cooling water. These ion-exchangers, which are generally synthetic organic resins, eventually become saturated with radioactive ions and must either be regenerated or removed from service and disposed of in a safe manner.
These ion-exchange resin wastes, as well as other low-level radioactive wastes are often disposed of by means of shallow land burial. Until recently, the majority of resin wastes have simply been dewatered and packaged in large carbon steel liners prior to shipment to the burial site. However at present the United States Nuclear Regulatory Commission (NRC) requires that all new commercial power plants licensed have the capability to solidify ion-exchange resin waste.
Low, intermediate and high level nuclear waste may also be disposed of into deep wells hundreds or thousands of feet deep. The borehole of these wells then must be sealed to prevent the wastes from rising and contaminating upper level ground water. Nuclear wastes are also disposed of by blending the waste with a mixture of cementitious materials and selected clays such as Grundite illite to form a grout. The grout is injected 800 to 1000 feet underground into cracks produced in shale bed formations by the pressure of the injected grout. Here the mixture solidifies, fixing the radionuclides. The clays in the grout act as an ion exchanger which prevent the radioactive ions, such as cesium, from being leached from the grout.
Studies have indicated that ion-exchange resin waste incorporated into portland cement exhibits poor waste performance. The waste forms have been shown to swell and crack, which will permit the entrance of ground water into the solid and diminish the benefits inherent in solidification. With any of the other methods of waste disposal, the possibility is always present that flowing ground-water will, over a period of time leach radioactive ions from the cementitious matrix. Of a particular problem is cesium ion which readily exchanges with calcium ions found in most ground water.
Tobermorite, a calcium silicate hydrate mineral was described in "The American Mineralogest", Vol. 51, March-April 1966, and incorporated herein by reference, as being produced by auto claving concrete or sand-lime masonry blocks, apparently serving as the principal binding agent in these building materials. Furthermore, it was found that there are several poorly-crystalline calcium silicate hydrates which are closely related to tobermorite. One of these is a tobermorite gel (or C-S-H gel) which is the basic cementing compound produced by the normal hydration of portland cement. For these reasons, tobermorite is believed to be very stable and compatible with cement.